Signaling through receptor/transcriptional regulator Notch plays a critical role in tumor cell survival. We have previously demonstrated that pharmacological inhibition of Notch pathway with GSI (\#947;-secretase inhibitor) induces apoptosis of multiple myeloma (MM) cells via dramatic up-regulation of proapoptotic BH3-only protein Noxa. Noxa mediates its effect via binding to antiapoptotic protein Mcl-1, but not Bcl-2 or Bcl-xL. Antiapoptotic members of bcl-2 family including Bcl-2 and Bcl-xL are known to promote tumor cell survival and chemoresistance and have been shown to be overexpressed by MM cells. Recently developed BH3 mimetic ABT-737 (Abbott) blocks Bcl-2 and Bcl-xL and might be effective in treatment of MM. Here we tested the possibility that inhibition of Notch signaling could substantially enhance pro-apoptotic effect of Bcl-2/Bcl-xL inhibitor ABT-737. The effect of GSI, ABT-737 or their combination was evaluated in four different MM cell lines and primary MM cells isolated from BM of patients with MM. Treatment with each compound alone resulted in MM cell death. Addition of GSI to ABT-737 significantly increased anti-MM cytotoxicity. The synergistic effect of this drug combination was mediated by activation of Bak and Bax and release of cytochrome c. When toxic for MM cells, combination of GSI and ABT-737 did not affect survival of peripheral blood mononuclear cells isolated from healthy donors. To test the effect of ABT-737 in combination with GSI on MM cells in vivo we used SCID/NOD mouse model. Mice were injected s.c. with human MM RPMI-8226 cells. Tumor became visible in ~3 weeks after inoculation and grew as a solid tumor which allowed easily monitoring tumor size. Mice were treated with ABT-737 (75 mg/kg/day, i.p., 9 days), GSI (5mg/kg/day i.p. for 9 days), combination of ABT-737 and GSI, or vehicle control. Tumor size was constantly monitored during treatment and for 4 weeks after the end of the treatment. Treatment with ABT-737 and GSI alone resulted in slight decrease in tumor burden as compared with control group. In contrast, combination of ABT-737 and GSI induced dramatic antitumor effect. Thus, our results demonstrated that inhibition of Notch enhances pro-apoptotic effect of Bcl-2/Bcl-xL inhibitor ABT-737 in vitro and in vivo and therefore this drug combination could have potential therapeutic benefit for treatment of MM.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2049.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO